Your boiler is undergoing a brief, normal startup process where the internal pump circulates water before the burner ignites.

pump pre-run

What causes this fault?

This code appears when the boiler's internal computer tells the pump to start moving water through the system before the gas is ignited. It is a standard safety and efficiency check designed to ensure there is enough flow to carry heat away from the heat exchanger. This short delay is a normal part of the ignition sequence and does not mean anything has broken.